Powder metallurgical techniques enable producing parts having complicated shapes in shapes (i.e. near net shapes) extremely close to product shapes, with high dimensional accuracy. The use of powder metallurgical techniques in producing parts therefore contributes to significantly lower machining costs. For this reason, powder metallurgical products obtained by powder metallurgical techniques have been used as various mechanical parts in many fields.
Powder metallurgical techniques mainly use iron-based powders. Iron-based powders are categorized into iron powder (e.g. pure iron powder), alloy steel powder, and the like, depending on the components. Iron-based powders are also categorized into atomized iron powder, reduced iron powder, and the like, depending on the production method. In the case of using the categories by the production method, the term “iron powder” has a broad meaning encompassing not only pure iron powder but also alloy steel powder.
Such an iron-based powder is used to produce a green compact. A green compact is typically produced by mixing an iron-based powder with alloying powders such as a Cu powder and a graphite powder and a lubricant such as stearic acid or lithium stearate to obtain an iron-based mixed powder, and then charging the iron-based mixed powder into a die and pressing it.
The density of a green compact obtained by a typical powder metallurgy process is normally about 6.6 Mg/m3 to 7.1 Mg/m3. The green compact is then sintered to form a sintered body. The sintered body is further subjected to optional sizing and machining work to form a powder metallurgical product.
In the case where higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.
Increases in strength of powder metallurgical products have been strongly requested recently, for reductions in size and weight of parts. There has been particularly strong demand for strengthening iron-based powder products (iron-based sintered bodies) made from iron-based powders.
Known examples of an iron-based powder as a powder with alloying elements added thereto at the stage of a precursor powder include: (1) a mixed powder obtained by adding each alloying element powder to a pure iron powder; (2) a pre-alloyed steel powder obtained by completely alloying each element; and (3) a partial diffusion alloy steel powder (also referred to as “composite alloy steel powder”) obtained by partially diffusionally adhering each alloying element powder to the surface of a pure iron powder or pre-alloyed steel powder.
The mixed powder (1) obtained by adding each alloying element powder to a pure iron powder is advantageous in that high compressibility equivalent to that of a pure iron powder is ensured.
With the mixed powder (1), however, matrix strengthening necessary to obtain higher strength may be unable to be achieved because each alloying element does not sufficiently diffuse in Fe during sintering and the microstructure tends to remain non-uniform. Besides, in the case of adding Mn, Cr, V, Si, etc. which are metals more active than Fe, unless the CO2 concentration and dew point in the sintering atmosphere or carburizing atmosphere are strictly controlled to low level, the sintered body oxidizes, and lower oxygen content in the sintered body necessary for strengthening the sintered body cannot be achieved.
Hence, the mixed powder (1) obtained by adding each alloying element powder to a pure iron powder has not been used due to its failure to cope with the recent requests for strengthening.
With the pre-alloyed steel powder (2), on the other hand, uniform microstructure can be obtained because the segregation of the alloying element is completely prevented. This contributes to stable mechanical properties. The pre-alloyed steel powder (2) is also advantageous in that, even in the case of using Mn, Cr, V, Si, etc. as alloying elements, lower oxygen content in the sintered body can be achieved by limiting the types and amounts of such alloying elements.
However, since the pre-alloyed steel powder is produced by atomizing molten steel, oxidation of the molten steel in the atomizing step and solid solution hardening due to complete alloying tend to occur. This hinders an increase in green density during press forming.
The partial diffusion alloy steel powder (3) is produced by adding each metal powder to a pure iron powder or a pre-alloyed steel powder and heating the resultant powder in a non-oxidizing or reducing atmosphere to partially diffusionally bond the metal powder to the surface of the pure iron powder or pre-alloyed steel powder. This partial diffusion alloy steel powder combines the advantages of the iron-based mixed powder (1) and pre-alloyed steel powder (2), while avoiding various problems seen in the iron-based mixed powder (1) and the pre-alloyed steel powder (2).
In detail, the partial diffusion alloy steel powder (3) ensures lower oxygen content in the sintered body and high compressibility equivalent to that of a pure iron powder. Moreover, since a multi-phase made up of a complete alloy phase and a partially concentrated phase is formed, the matrix can be strengthened. The partial diffusion alloy steel powder has therefore been widely developed as it can cope with the recent requests for strengthening parts.
Basic alloy components used in the partial diffusion alloy steel powder include Ni and Mo.
Ni enables a large amount of non-transformed austenite phase that does not form quenched microstructure even when quenched, to be retained in the metallic microstructure. Ni is known to have an effect of improving the toughness of parts and solid-solution-strengthening the matrix phase by this action.
Mo has an effect of increasing hardenability, and so suppresses the formation of ferrite during quenching and facilitates the formation of bainite or martensite in the metallic microstructure. By this effect, Mo not only transformation-strengthens the matrix phase, but also solid-solution-strengthening the matrix phase by dispersing in the matrix phase, and forms fine carbide in the matrix phase to strengthen the matrix phase by precipitation. Mo also has good gas carburizing property and is a non-grain boundary oxidizable element, and so can strengthen the sintered body by carburizing.
As an example of a mixed powder for high strength sintered parts using a partial diffusion alloy steel powder containing these alloy components, JP 3663929 B (PTL 1) describes a mixed powder for high strength sintered parts obtained by mixing an alloy steel powder formed by partially alloying Ni: 0.5 mass % to 4 mass % and Mo: 0.5 mass % to 5 mass % with Ni: 1 mass % to 5 mass %, Cu: 0.5 mass % to 4 mass %, and a graphite powder: 0.2 mass % to 0.9 mass %.
As an example of an iron-based sintered body having high density and not containing Ni, JP H4-285141 A (PTL 2) describes a method of producing an iron-based sintered body by mixing an iron-based powder of 1 μm to 18 μm in mean particle size with a Cu powder of 1 μm to 18 μm in mean particle size at a weight ratio of 100:(0.2 to 5) and forming and sintering the mixed powder.
This technique uses an iron-based powder having an extremely smaller mean particle size than a typical iron-based powder, and thus achieves a high sintered body density of 7.42 g/cm3 or more which is normally impossible.